Glass optical waveguide fibers are remarkably strong as they are drawn, but their strength is greatly affected by physical handling. They must therefore be coated with a protective material before they come in contact with the fiber drawing tractors. For a number of reasons including improved diameter control and cleanliness, the tractors should be as close as possible to the draw furnace. At a fixed distance from the furnace, the fiber becomes hotter at higher draw rates. Thus, to cool a fiber by natural cooling, longer distances are required for higher draw rates. For example, to cool a 125 .mu.m diameter fiber from 1780.degree. C. to a temperature needed to apply a cellulose acetate lacquer solution with acetone, approximately 80 cm of cooling distance is required for a draw speed of 0.5 m/sec. A distance of 120 cm is required to cool that fiber to 50.degree. C. at a draw rate of 0.75 m/sec., and 800 cm is required if the draw rate is 5 m/sec.
The economical production of large quantities of optical waveguide fibers will undoubtedly require fiber drawing speeds greater than 1 m/sec. As fiber drawing speeds approach 1 m/sec., presently constituted fiber drawing systems do not afford sufficient time for the fiber to cool by natural processes to temperatures which are compatible with application techniques employing presently developed waveguide coating materials. When the temperature of the fiber is too high, the coating may become too thin or discontinuous. If the coating is applied from a 100% solids solution, the hot fiber can cause the coating material to set up in the coating apparatus around the fiber, thus preventing any further coating material from being applied to the fiber.
The optical waveguide fiber coating system disclosed in U.S. Pat. No. 4,208,200 employs means for cooling the hot fiber prior to its entering the coating apparatus. The cooling means comprises an elongated, liquid filled container through which the fiber passes. The bottom of the container is provided with a felt wiping die which seals the container and removes excess liquid from the fiber. This type of fiber cooling device has a number of disadvantages. At high draw rates the fiber can become so hot that it boils the liquid. This causes turbulence which can move the fiber laterally so that it is displaced from its proper position in the diameter measuring device. Also, the wiping means physically contacts the fiber, a factor which might adversely affect fiber strength.